This application is based upon and claims priority from prior French Patent Application No. 0008215, filed on Jun. 27, 2000, the entire disclosure of which is herein incorporated by reference.
1. Field of the Invention
The present invention generally relates to the field of monostable circuits, and in particular to monostable circuits used for managing redundancies in memories.
2. Description of the Prior Art
Memories are generally constructed as a matrix of rows and columns of memory cells. For a memory of given size, a certain number of additional rows and a certain number of additional columns are provided, intended to replace rows or columns whose post-fabrication operational tests proved to be negative.
This operation of replacing one or more rows and one or more columns requires the reconfiguring of the memory or more generally of the circuit to be repaired. However, this reconfiguring must not degrade the performance of the circuit either in terms of consumption or in terms of speed. Therefore, use is made of fuses that can be placed in open circuit by the firing of a laser beam. A circuit making it possible to recognize the state thereof, open or closed, and to provide digital information relating to this state must be arranged in proximity to the fuse. It goes without saying that this circuit must also be of small size and have low power consumption.
The testing of a memory generally consists in addressing all the cells in write mode according to a predetermined template, then in read mode so as to establish a map (bitmap) of the memory. The map obtained is compared with the template and any anomalies are thus detected. The blowing (the placing in open circuit) of the fuses is then organized so as to modify the interpretation of the addressing of the defective columns.
The means of blowing the fuses depend on the nature of the fuses used. In the case of laser-blowable fuses, all the fuses associated with the replacement of defective columns of all the circuits made on one and the same integrated circuit board are blown in succession by means of a laser.
Accordingly, there exists a need for overcoming the disadvantages of the prior art as discussed above.
The present invention proposes a circuit that achieves a reliable interface between the fuse and a digital output.
The present invention further proposes a circuit that is hardly sensitive to disturbances, in particular those due to alpha rays or to heavy ions.
A blowable circuit with digital output, according to one aspect of the invention, comprises an auto-stable assembly of latches, a control assembly, a blowable assembly, a logic gate comprising a first input coupled to a common point between the auto-stable assembly and the blowable assembly, and a second input coupled to the control input of the circuit, and a breaker controlled by the output of the logic gate and arranged between the auto-stable assembly and ground. The auto-stable assembly allows the information to be preserved despite harsh environmental conditions, due for example to incident alpha rays or heavy ions.
In one embodiment of the invention, the blowable assembly comprises a blowable element and a breaker which are arranged in series between the auto-stable assembly and a ground.
In one embodiment of the invention, the blowable element is coupled to the auto-stable assembly.
In another embodiment of the invention, the breaker of the blowable assembly is coupled to the auto-stable assembly.
Advantageously, the logic gate is of NAND type.
In one embodiment of the invention, the control assembly comprises two control breakers. The control input of the circuit can be coupled to the control terminal of the said two control breakers. Two inverters can be arranged between the input of the circuit and the control terminal of one of the said control breakers.
In one embodiment of the invention, the auto-stable assembly comprises at least four breakers, preferably eight breakers.
The present invention also relates to a process for reading a blowable element, in which:
before blowing, a control input coupled to a first input of a logic gate being at a first logic value, breakers are off, imposing in the second input of the said logic gate a logic value such that the output is at a complementary logic value,
before blowing, the said control input being at the logic value complementary to the first logic value does not modify the output, the logic value of the second input of the logic gate being maintained,
after blowing, the control input coupled to the first input of the said logic gate being at a first logic value, the breakers coupled to the input of the circuit are off, imposing on the second input of the said logic gate a logic value such that the output is maintained in the said complementary logic value and that, after the said control input has received a complementary logic value, the output goes permanently to the said first logic value.
By virtue of the invention, one obtains a circuit that is compact, that is to say occupies a small area of silicon, robust, and of low consumption irrespective of the state, open or closed, of the blowable element.